1. Field of the Invention
This invention relates to aliphatic polyisocyanates containing ester groups which combine particularly high functionality with extremely low viscosity, to a process for the production of these polyisocyanates and to their use as the isocyanate component in two-component polyurethane coating compositions.
2. Description of the Prior Art
Relatively high functionality polyisocyanates containing aliphatically bound isocyanate groups have acquired considerable economic and industrial significance as the isocyanate component for two-component polyurethane coating compositions. Known polyisocyanates for use in polyurethane coating compositions are generally produced by biuretization, trimerization or urethanization of low molecular weight diisocyanates, particularly hexamethylene diisocyanate.
There are several known processes for the production of aliphatic biuret polyisocyanates. Monomer-free, commercially available polyisocyanates produced by biuretization generally have average functionalities of 3 to 4. The products having a high functionality, i.e., products having a functionality of about 4, inevitably have a higher viscosity than those of low functionality, i.e. products having a functionality of about 3.
Biuret polyisocyanates having a particularly low viscosity are described, for example, in US-PS 3,903,127. The interdependence of functionality and viscosity is impressively demonstrated by Example 3 of this patent. The polyisocyanate having a functionality of 3 has a viscosity of 750 mPa.s/20.degree. C. (Example 3e). If the mixture contains 18% products of higher functionality, its viscosity rises to 1,350 mPa.s/20.degree. C. (Example 3b). If the percentage of products of higher functionality increases to 28%, the mixture has a viscosity of 2,560 mPa.s/20.degree. C. (Example 3a).
The production of isocyanurate polyisocyanates is also described in numerous publications, the conditions are similar. Again, the products with higher functionalities also have higher viscosities. However, the isocyanurate polyisocyanates having an average functionality of 3 to 4 have a somewhat lower viscosity than the biuret polyisocyanates having a comparable functionality. DE-OS 3 810 908, for example, describes isocyanurate polyisocyanates of particularly low viscosity. If the oligomer mixture contains 30% of the products having a functionality of greater than 3, its viscosity is 1,300 mPa.s/25.degree. C. (Example I). If the mixture contains 25% of products having a functionality of greater than 3, its viscosity is only 1,000 mPa.s/25.degree. C. (Example 2).
The production of polyisocyanates for polyurethane coating compositions by urethanization of low molecular weight diisocyanates with polyhydric alcohols results in high viscosity to resin-like products which can only be processed by the addition of organic solvents.
Considerably lower viscosities are only shown by polyisocyanates containing a high percentage of uretdione structures also possess considerably lower viscosities, as described for example in U.S. Pat. No. 4,614,785. However, these polyisocyanates generally have an average functionality below 3, which means that the low viscosity is accompanied by the disadvantage of a low functionality.
The average NCO functionality is of considerable importance in regard to the suitability of the polyisocyanates for the production of high-quality coatings. The higher the functionality, the higher the crosslink density in the coating and, thus, the higher its resistance. However, the higher viscosities associated with the high functionalities are a disadvantage because they necessitate the increased use of organic solvents for establishing the necessary application viscosities.
Accordingly, it is an object of the present invention to provide new polyisocyanates which combine high functionality with low viscosity, so that they may be used for the production of high-quality, low-solvent or solventless two-component polyurethane coating compositions.
This object was achieved in accordance with the present invention by the polyisocyanates described in detail hereinafter. The new polyisocyanates according to the invention have a viscosity of at most 2,500 mPa.s/22.degree. C., preferably at most 1,000 mPa.s/22.degree. C., and an average functionality of at least 4.1, preferably at least 4.5 and more preferably at least 5.
In terms of chemical structure, the new polyisocyanates contain ester groups, have an average molecular weight below 2,000 and an average functionality of at least 4.1.
Polyisocyanates containing ester groups are known. DE-OS 2 120 090 describes polyisocyanate prepolymers which contain ester groups, have an average molecular weight of about 900 to 20,000 and are suitable for the finishing of textiles. These products are applied from dilute solutions. They are not suitable for the formulation of low-solvent two-component coating compositions. DE-OS 3 634 248 describes a process for the production of polyisocyanates which contain ester groups. However, the polyisocyanates described in DE-OS 3 634 248 do not have the high functionality required by the polyisocyanates of the present invention because the polyisocyanates containing ester groups are produced solely by reaction of alcohols having a maximum functionality of 4 to form polyisocyanates which have a functionality which corresponds to the functionality of the polyhydric alcohols from which they are prepared. This reference does not disclose polyisocyanate mixtures based on tetrahydric alcohols which, in addition to the corresponding tetrafunctional polyisocyanates, also contain homologs of higher functionality such that the average functionality is at least 4.1, nor does the reference disclose a process for the production of high functionality, low viscosity polyisocyanates containing ester groups based on polyols having a functionality of 5 or higher.